1. Field of the Invention
The present invention relates to a capacitive electro-mechanical transducer, and a fabrication method of the capacitive electro-mechanical transducer.
2. Related Background Art
Conventionally, an ultrasound transducer has been used as the ultrasound probe of the ultrasound medical diagnostic apparatus. The ultrasound transducer is a device for converting an electric signal into an ultrasonic oscillation, or vice versa. In the ultrasound medical diagnostic apparatus, an input electric signal is converted into ultrasound to be transmitted into a living body, and the ultrasound reflected in the living body is received to be converted into the electric signal. As a type of the ultrasound transducer, there is the capacitive electro-mechanical transducer. The capacitive electro-mechanical transducer includes a space or a so-called cavity that is maintained under an approximately evacuated condition, and two electrodes sandwiching the cavity therebetween. One of the electrodes is arranged on a thin film or a so-called membrane, and is movably supported.
At the time of an ultrasound transmitting operation, the electric signal according to the ultrasound is applied between the two electrodes. At this time, the membrane and the electrode disposed thereon generate the ultrasonic oscillation due to electrostatic attractive force acting between the two electrodes. On the other hand, at the time of an ultrasound receiving operation, the membrane and the electrode thereon oscillate due to the ultrasonic oscillation from outside. At this time, a distance between the two electrodes changes according to the oscillation, so that the electric capacitance between the two electrodes changes. This capacitive change is detected as the electric signal. Technologies of the capacitive electro-mechanical transducer are disclosed in U.S. Patent Application No. 2008/089179 A1 and U.S. Pat. No. 5,982,709.
A fabrication method of the capacitive electro-mechanical transducer includes a step of forming a sacrificial layer in the form of a cavity on a substrate, a step of forming a membrane layer on the sacrificial layer, and a step of etching the sacrificial layer to form the cavity. An etching hole in the membrane layer is formed beforehand prior to the step of etching the sacrificial layer to supply an etching liquid from outside. Further, the etching hole in an approximately vacuum ambience is closed after the step of etching the sacrificial layer to seal the cavity from outside.
In the capacitive electro-mechanical transducer described above, a degree of vacuum in the cavity influences transmit output characteristics and receive sensitivity characteristics of ultrasound. Generally, when the degree of vacuum in the cavity decreases and gas or liquid intrudes into the cavity, the damping effect acts on the membrane. Hence, transmit output characteristics and receive sensitivity characteristics vary. Therefore, in such a capacitive electro-mechanical transducer, it is important to maintain the degree of vacuum in the cavity for the stability of device characteristics. Even if the cavity structure is damaged for some reason, a decrease in the degree of vacuum in the cavity is suppressed within a minimum range. In the capacitive electro-mechanical transducer described above, however, adjacent cavities communicate with each other through an etchant flow path or a communicating portion formed for the etching of the sacrificial layer. Consequently, when the membrane or vacuum sealing portion in a portion of the cavities is damaged, a vacuum sealing condition of all the cavities communicating with each other is liable to be affected.